1. Field of the Invention
This invention relates generally to the field of welding power supplies and, more particularly, to an improved welding power supply incorporating a series resonant converter which is capable of producing a range of constant current outputs for pulsed arc welding and which is stable in operation at all output values.
2. Description of the Prior Art
Power sources for arc welding can be designed on the "inverter" theory. Among the various topologies is the series resonant converter (SRC) operating at high conversion frequencies to save weight and size. However, it is well-known that, particularly when an SRC is asked to produce a range of constant current outputs, stable operation at all output values is difficult to achieve. Furthermore, prior art SRCs generally required at their output large capacitors which become part of the tank "loop" of the SRC. Such capacitors are undesirable from a welding power source point of view, especially for fast response power sources which are expected to provide a dynamic pulse output.
More specifically, in the prior SRC art, particularly as disclosed in U.S. Pat. No. 3,953,779--Schwarz, gapless cores (toroids) are used for the transformer, and there is required at the output a large capacitor (for example, capacitor C.sub.o in FIG. 1 of the Schwarz patent) with high ripple current capability but, preferably, with a moderate capacitance in .mu.F's. The high ripple current requirement comes from the carrier (resonant) frequency current of the SRC, the current induced by the dynamics of the load and the pulse frequencies when pulsed GMAW welding is performed. Excess ripple current due to these sources caused excess heating of the output capacitor which is generally chosen as an electrolytic type in the interest of cost. Welding applications must contend with these high ripple conditions and, thus, the Schwarz converter is not usable in welding applications for this reason and for the following additional reasons:
1. In welding machines, particularly for use in the United States, it is customary to provide dual-voltage primary connections for the welding power source in order to accommodate both 230 and 460 volt three-phase inputs which, hen rectified, produce d.c. voltages of 320 and 640 volts, respectively. A method of reconnection to accommodate the two input voltages is to provide two bridges, then to connect them in parallel when the lower primary voltage is to be used, and in series when the higher primary voltage is to be used. However, the two bridges must balance and share equally in providing the load current. If they do not equally share the load, a voltage unbalance will occur in the two electrolytic input capacitors. The unbalance cannot be corrected in an analog manner because the two converters are operated phase-displaced and cannot be operated at different gating frequencies. To correct unbalance with heavy passive bleeder resistors is prohibitively wasteful of energy and creates a heat problem, etc. When the bridges of the two SRCs do not balance, voltage builds up on one of the bridge inputs and diminishes on the other, a condition which can produce a catastrophic voltage level which results in capacitor failure.
2. D.C. offset on the tank capacitor causes instability when a loss of ring back occurs, and transformer saturation is likely to follow this loss of ring back. Furthermore, an asymmetry of the waveform occurs (similar to synchronous noise), and the system becomes unstable and must be stopped or reset.
3. The system will not feed high voltage loads, such as 55 to 60 volt arcs. Special (and costly) boost circuitry is necessary instantaneously to "change the transfer turns ratio" in order to feed momentary high-voltage loads.
4. A "foot" develops on the SRC's volt-ampere curve because of free wheeling of the energy in the transformer's leakage inductance. This current is not monitored or is unrecognized by the primary current control system.
5. Prior art SRCs often use capacitors in series with the transformer secondaries in order to guarantee ring backs, along with full-wave diode bridge rectifiers. These capacitors must handle full load current. Such a prior art circuit is costly in components and doubles the rectifier losses when compared with a two-diode center-tapped arrangement, without capacitors.
There are several reasons why this d.c. offset can occur, but one of the principal reasons involves the output capacitor mentioned above:
1. With a dynamic arc load, fast cutback can occur, that is, the gating frequency of the switching SCRs can change from very high, where the tank is operated in the cut-in mode, to much lower where the tank is operated in the discontinuous conduction mode, in a short time. As an example, while welding, a drop of molten metal can bridge the gap between the electrode and the workpiece, thereby causing a change in the apparent arc voltage from 30 volts to near zero volts in a fraction of a millisecond. The regulating system detects this condition and quickly reduces the gating frequencies. However, this quick change in a downward direction can leave an offset voltage (an unwanted d.c. level) on the tank capacitor. This d.c. level diminishes the next ring forward half cycle, thereby reducing or eliminating the next ring back. An instability occurs because, with a loss of a ringback, transformer saturation occurs in the extreme. Another problem is that the SCR and tank capacitor voltages can become excessive at offset. Tank capacitors fail if they are subjected to voltage peaks well beyond their rating. An equally bad problem is that the d.c. offset can cause perturbations in the normally smooth output of the welding power source, triggered by rapid and dynamic changes in the arc load (which changes are perfectly normal and must be handled).
2. The Schwarz inverter requires boost circuits in order to feed loads in excess of 40 or 45 volts. The boost circuit consists of means for electronically switching the transformer turns ratio, thereby requiring additional cost and complexity, and introducing a destabilizing transient to the power circuit when it is called upon to operate.
3. The analog signal to discrete time interval converter ("asdtic"--see, for example, U.S. Pat. No. 3,659,184) or controller regulated only primary current, as used in the Schwarz type of inverter. Current transformers alone measured the primary current. The current transformers did not read the free-wheeling current in the d.c. secondary, and thus, did not truly reflect the actual output.
4. When an output electrolytic capacitor is used as proposed by Schwarz, the peak tank current decreases when slewing towards high repetition rates and, then, increases rapidly at "cut-in". Cut-in is the point where the SRC goes from discontinuous single sine waves to a continuous conduction wave train. This is a traumatic point in the range of the controller and results in discontinuities in the smooth output characteristics desired while sweeping from low current to high current output and vice versa. Also, when the dynamic load slews back and forth across the cut-in point, it induces additional problems due to the momentary d.c. tank offsets which were previously mentioned above.
5. In dual primary voltage welding machines (i.e., 230/460 volts), it is advantageous to provide two complete "H" bridges, and, then, connect them in parallel for the lower primary voltage and in series for the higher primary voltage. In the high primary voltage case, the input is rectified to a d.c. bus which is divided into two buses by a series capacitor arrangement. Each half bus then feeds one "H" bridge. These two inverters run simultaneously, and their d.c. outputs are paralleled for additive current. If there is an discrepancy in the current "draw" between the two inverters, the capacitive input divider can become unbalanced, to a point where one may be seriously over-voltaged while the other is well below its voltage rating. Such a discrepancy in current draw can produce a regenerative effect wherein the unbalance "runs away".
There are other U.S. patents which may be considered as relevant prior art.
More specifically, U.S. Pat. No. 4,520,255 discloses a self-oscillating welding circuit employing transistors as switches to cause current to flow alternately in opposite directions to the primary winding of a welding transformer.
U.S. Pat. Nos. 4,152,759 and 4,382,171 disclose frequency converters of a series capacitor type and used to supply welding current.
U.S. Pat. No. 4,117,303 discloses a frequency converter welding apparatus which eliminates problems, caused by an output capacitor, by using a choke which is placed upstream of an output capacitor to support the welding arc during a short-circuit condition.
U.S. Pat. Nos. 4,369,489 and 4,581,692 disclose d.c. voltage converters including transformers which are provided with an air gap but, apparently, not for the purpose of eliminating an output capacitor for use in arc welding.
U.S. Pat. No. 3,893,015 discloses an inverter power supply including two-series converters, together with transformer means for transferring the power from the a.c. output voltage of one inverter to the d.c. input of the other inverter, thereby forcing the two inverters equally to share the load.
The following patents may be of additional background interest: U.S. Pat. Nos. 3,621,362; 4,048,468; 4,200,830; 4,460,949 and 4,628,427.